Right now I’m wiring up a system that uses some of Pololu’s a4988 boards. I looked at the a4988 datasheet and it seems to say that the chip only uses 20 microamps of current max…even if the full board uses 1000x that, that’s still under the 40mA pin limit on the Atmega2560 chip I’m using (and I’m well within the 200mA total limit).
Edit: Rereading, it looks like the chip uses more like 8mA when operating. Still within limits, but no longer the huge margins I thought. Am I still ok?
So, it would save me a wire if instead of having an enable wire that turns the output fets on and off, I simply turning the Vlogin on and off for the board.
The thing is, this seems a very suspicious practice, and I’m not familiar with any problems I might face. Does anyone have suggestions? Am I allowed to do this without damaging either the microcontroller or the board?
By “Vlogin”, do you mean the board’s logic power, VDD? I think what you are proposing should generally be okay, but I recommend reviewing the datasheet carefully to make sure that there aren’t any warnings about supplying VMOT without powered logic. Also, you should make sure you never supply any of the inputs with 5V while the logic is unpowered (the ratings for the driver inputs are a function of VDD, so powering these inputs while the device is unpowered could break the device).
I do not know how much current the VDD line draws, but it should be easy for you to check if you have a multimeter (I would not go strictly by what the datasheet says since the carrier board has additional connections and components on it). You could also check the VDD voltage with your multimeter while you are powering the board from an I/O line to make sure it isn’t dropping too much from the expected 5V.
correct call on my silly typo. “Vlogin” was some accidental mash-up of “Vlogic/VDD”
Thank you for all your tips. I think those are perfect suggestions and exactly what I wouldn’t know but needed to know (like, not supplying 5v to the inputs while there is no logic voltage.)